Prior to this commit, Spring Boot would auto-configure Spring MVC and
would keep the default `UrlPathHelper` configuration.
Since Spring Boot is in charge of configuring the `DispatcherServlet`
and its mapping, it is in a position to optimally configure the
`UrlPathHelper` depending on the chosen mapping.

This commit sets the `alwaysUseFullPath` property of `UrlPathHelper` if
the Servlet mapping is `"/"`. This is more efficient since this
configuration requires less processing of the request path.

Closes gh-21499